Re: decay
Kevin:
You cannot always catch decay in the tree by examining the face. It could be elsewhere, this would most commonly be the case in face cuts that are shallower than 1/3rd in.
Mostly the reason for boring vertically into trees that you suspect rot is to understand any weakness before you start your notch. With rot not always being balanced nor uniform in amount, understanding these flaws needs to be part of the fallers decision process on what can/should be done with a tree/snag.